Effects of diversity, topography, and interannual climate variability on pathogen spillover

Text from an abstract I submitted for the 6th Sudden Oak Death Science Symposium on the initial results from one of my dissertation chapters. Additional analysis is in progress.

Our knowledge of sudden oak death (SOD) disease dynamics indicate that without bay laurel (Umbellularia californica) there is seldom oak (Quercus) infection. This requirement of an alternate host species for disease transmission to oak species is an example of pathogen spillover. We developed a path analysis to test specific hypothesized relationships between physical and ecological factors affecting pathogen spillover. Path analysis enables simultaneous examination of direct and indirect effects from multiple factors, which can enhance our understanding of the multiple influences on pathogen spillover in SOD. We rooted our path model with the topographic wetness index, indicating potential soil wetness and moisture persistence, and examined the direct and indirect effects of species diversity, temperature, precipitation, and bay laurel density on potential inoculum load and infection of oak species.


Path model structure defining relationships between factors influencing pathogen spillover.

We applied 10 years of data from a long-term SOD-monitoring plot network in southeastern Sonoma County, CA. Each of the 200 15-m by 15-m plots was equipped with a temperature logger and plots were visited once per year from 2004 to 2012, and in 2014 to assess P.ramorum/SOD host species for disease symptoms and download temperature data. We inspected oak species for canker symptoms and indexed potential inoculum load by counting symptomatic leaves on each bay laurel stem for 60-seconds. We recorded the abundance of all tree species rooted in each plot during visits in 2005 and 2014 to quantify community diversity. Rainfall was measured at 15 rain gauges installed throughout the study area during this period.


A field crew member taking measurements at one of the plots.

We conducted a piecewise assessment of the path model, enabling us to account for the repeated measures structure of these data. Results from our path model of disease observations aggregated to the plot level revealed that diversity mediates the potential for pathogen spillover through a relatively strong direct negative effect on oak infection. Potential inoculum load on bay laurel had a direct positive effect on oak infection, with its overall influence moderated by temperature, topography, and diversity. Temperature and rainfall had relatively weaker influences on pathogen spillover compared to diversity and inoculum load. The net negative effect of diversity on oak infection is consistent with the dilution effect found in other studies of SOD. Topographic wetness had significant direct influence on diversity and inoculum load, where higher values of the wetness index tended to have lower values for diversity, but higher values for inoculum load. This is consistent with areas where moisture is likely to accumulate and persist providing a more favorable environment for P. ramorum sporulation.


Our frontier is climate change

UPDATE: This link to recent research showing a link between human CO2 emissions and the increased water vapor in the troposphere — possibly causing a positive feedback to greater warming.

Last week one of the scientific contributors to the recently published IPCC (Intergovernmental Panel on Climate Change) ipcc_ar55th Assessment Report, who is also an alumnus of NC State. In addition to the associated prestige, the speaker found that the force driving many of the contributors to spend countless hours working on this assessment is the hope that our global population will become sustainable. To me, this means providing avenues for equity not just in this generation, but for future generations. So, what would we have to do? Well, we can start by communicating the facts, so here are some things I learned and what I think they mean.

1. According to the analyses in 5th Assessment,  there is a 95% certainty that humans have been the dominant cause of observed warming since the mid-20th century. To me this is incredibly important, a little bit terrifying, and yet gives me hope. This means that the global human population is capable of affecting the earth systems to a degree never before seen or realized. It means that we hold some power to influence global cycles, and we can use this knowledge and our incredible ability to forecast, innovate, and adapt to sustain and prosper our existence.

2. One of the most, if not the most, important earth cycle we are affecting is the water cycle. Human bodies are made up of about 60% water. Water that is fit for consumption is the one thing we cannot live with out – something that is seldom given a second thought in the developed world. We just turn on the tap and out it comes. Yet, we have with great certainty (95%) increased the frequency and severity of drought or precipitation events in different areas of the globe (keeping in mind that no single event is attributable to human-induced changes).

3. The increase in carbon dioxide emissions caused by human’s activities were identified as one of the major influences on the rapid changes in earth’s systems. Furthermore, even if CO2 emissions were completely stopped, the inertia would continue to carry changes into the future. It is not as simple as a single consequence of increased CO2 concentrations in the atmosphere, such as warming temperatures, but also higher CO2 concentrations in the oceans causing acidification faster than species can adapt. This means that the livelihood of everyone who depends on the ocean is at increasing risk going into the future.

4. These are some indications that we will continue to exceed (mostly unidentified) resilience thresholds, where the systems we are influencing will no longer be able to self-correct. Populations of developing countries have thus far born the brunt of the consequences, with >95% of deaths related to natural disasters occurring in these locations (I do not know if this included earthquakes, or other things not related to climate/weather events).

So, with a 95% certainty that we can potentially have some control over the intensity and severity of future risks to much of the global population, why not try and do something to prevent and mitigate that risk? As a disease ecologist, it is clear to me that the cost of preventative action (e.g. vaccines) is substantially less than trying to manage the likely severe consequences (e.g. disease outbreak).